The Centers for Disease Control and Prevention this week tallied 21 more cases of mpox, the disease, as authorities are now bracing for the possibility of a .
Illinois makes up the largest share of new cases, with nine new infections reported to the CDC. New York and Maryland each counted three additional infections.
Mpox case totals for Alabama, Arizona, Florida, Louisiana, Texas, Utah and Virginia each climbed by one since last week.
The agency recently deployed staff to investigate a number of cases around Chicago, which includes several infections of fully vaccinated men. Health officials say the outbreak has primarily involved men who have sex with men.
"CDC is collaborating with the Chicago Department of Health to investigate this cluster and specifically looking into why we are seeing more vaccinated cases than expected," Dr. Christopher Braden, head of the CDC's mpox response, said in a recent call with clinicians.
One theory being investigated is that the virus may have evolved mutations to evade the two-dose Jynneos vaccines that were rolled out last year to protect against it.
It is unclear when samples from the cases will be sequenced. A spokesperson for the Chicago Department of Public Health did not respond to a request for comment.
In an unpublished study that has yet to be peer-reviewed, the CDC reported monkeypox strains had turned up with signs of resistance to tecovirimat, the main antiviral drug used to treat patients. One of those worrying mutations was also detected "in a cluster of cases" around Los Angeles. That suggests drug-resistant monkeypox can be transmitted person-to-person in at least "rare cases."
Overall, the U.S. is still averaging less than three cases reported each day, far smaller than the hundreds of infections reported over the summer last year.
In total, 32 infections have so far been diagnosed for the month of May. Last month had reached a sum of 41 mpox cases.
But authorities have warned for months of the looming risk of a resurgence this year among communities at the highest risk of exposure, like men who have sex with men, starting with travel and festivities around Pride Month in June.
"The risk of near-term clusters and outbreaks is substantial and the outbreaks could be large, especially in the warmer months, with planned gatherings that may have the potential for skin-to-skin contact and increased sexual activity," Braden said.
Thealso prompted the Biden administration to begin weighing whether a shift in vaccine strategy — like additional boosters — might soon be needed.
"We've already, really immediately after seeing the Chicago cluster, convened folks within the U.S. government to discuss what the data is that we have and if there needs to be any change," the White House's Dr. Demetre Daskalakis told reporters last week.
For now, Daskalakis said the CDC is still urging at-risk Americans to only get the initial two doses of Jynneos vaccine.
CDC modeling suggests the vaccine could still blunt the size of a resurgence, even if immunity begins to wane. Studies released last week suggest two doses were between 66% and 86% effective earlier in the outbreak.
Ongoing studies of vaccinated residents in California and Washington, D.C., as well as findings from the investigation in Chicago, might open the door for more shots.
"We are on it, from the perspective of having the scientific discussions and are obviously, as sort of demonstrated in the track record of the response, really adjust our strategy based on what science is showing us," said Daskalakis.
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